Yup, you just need to edit the PedFiles for the levels you want to add more peds to. They're just XML files which define where peds spawn, what types of peds, how many, etc.

Best to run the game with extracted data, you can use Minge to extract the data and launch the game with ZADs disabled. With all the data extracted you can find PedFiles in the relevant level's folder inside \Data_Core\Content\Levels\

I tried this out as you suggested, and while it did in fact add lots more peds in the area I was trying to add peds to, it also removed all the ped sounds? Not sure what I could've messed up, is this common?

Edit: I think it’s due to the “disable Zads” command in Minge. Not sure why it’d be causing only ped sounds to stop.